The present invention relates to web-forming methods and apparatus particularly designed for the handling of cellulose pulp.
In sheet-forming and dewatering machines of this type, after the web-forming operation, the resulting pulp sheet is first wet-pressed and then dewatered either by means of drying cylinders or in a blower dryer. The wet end, or the web-forming part of the machine, is conventionally of the well known fourdrinier design having a flat wire on which the sheet forms. On such a flat wire the cellulose pulp web which is to be manufactured usually has a weight of about 600-1000 g/m.sup.2, and the web which is formed can be endowed with structural and strength characteristics which in the first place are appropriate with respect to the operation of the drying machine.
It is well known that when paper is manufactured with a fourdrinier, the continuous velocity of the pulp slurry jet which discharges from the slice of the headbox onto the travelling flat wire may differ appreciably from the speed of the wire. Thus, the speed of travel of the pulp slurry jet may be as much as 20% above or below the speed of travel of the wire, without any major difficulties resulting from this differential in the operation of the machine itself. However, the velocity differential is normally plus or minus 5%. On the other hand, the characteristics of the paper itself, for example the uniformity of the paper structure or the cloudiness thereof, or the ratio of the cross-machine and machine-direction strengths of the paper, are greatly dependent upon the velocity ratio, as has been set forth e.g. in the Journal Paperi ja Puu, 1967, No. 4a, page 148.
On the other hand, when a cellulose web is formed with a fourdrinier wet end, it has been recognized that the velocity of the pulp slurry jet must of necessity always be considerably in excess of the linear travelling speed of the wire, with the differential in this case being up to twice the wire speed, so that it is possible to achieve in this way a pulp web capable of travelling in a zig-zag path first in one direction and then the other around and between rolls and cylinders without breaking. If it is attempted to form a cellulose web at the same velocity ratio as in paper-making operations referred to above, it is not possible for the cellulose web to travel through the machine without breaking. It appears that only as a result of the excess velocity of the unbeaten cellulose fibers can the latter form a pulp web of sufficient solidity to tolerate the stresses to which the web is subjected, for example, on drying cylinders and rolls. It may be mentioned in this connection that the thickness of a pulp web formed on a flat fourdrinier wire is usually on an order which will provide a weight of 1200 g/m.sup.2 at the most.
It is known to use as a cellulose sheet-forming machine, for handling wet slurries, a Kamyr lifting cylinder, or in other words instead of a fourdrinier machine a cylinder machine where a mold cylinder rotates in a vat which contains the pulp slurry. A wet end of this type is also used under certain circumstances with blower dryers, particularly when forming very thick webs which have a weight of between 1500 and 2000 g/m.sup.1. In order to form relatively thin cellulose webs, however, having a weight of less than 900 g/m.sup.1, the Kamyr lifting cylinder is not suitable, primarily because of difficulties encountered in the pressing phase which follows the web-forming phase. It should be noted in general that the rotary lifting or mold cylinder rotates in a vat or basin which is filled with pulp slurry, and therefore the possibilities for controlling web-formation are rather restricted, so that the Kamyr type of machine is not able to operate under running conditions according to which factors such as the cellulose pulp speed and base weight can be selected within wide limits.